Hand dryers are typically installed in public washrooms as an alternative to paper towels.
There are three main types of hand dryer on the market: “warm-air” hand dryers, “high speed” hand dryers and “air-knife” hand dryers.
Warm air hand dryers are very well known. They are invariably low flow, low speed machines which rely on heating the air to promote an evaporative drying effect at the surface of the hand. Examples include the Model A Series of hand dryers manufactured and marketed by World Dryer Corporation. The heated airflow is typically discharged through a single nozzle and the drying action is a “hand-over-hand” action, requiring the user to rub the hands together under the nozzle with the aim of encouraging the evaporative drying effect.
High speed hand dryers, as the name suggests, use high speed airflow (>80 m/s) to provide a momentum-drying effect at the surface of the hands. Examples include the Xlerator® hand dryer manufactured and marketed by Excel Dryer Inc. Again the airflow is typically discharged through a single nozzle and the mode of use is somewhat similar to the “hand-over-hand” action of the warm air dryer, with the hands being held or cupped together underneath the nozzle to dry them. However, instead of being evaporated, the vast majority of the water on the surface of the hands is instead driven or blasted from the hands by the high-momentum airflow, with evaporation accounting for only a small proportion of water removal. The airflow tends not to be heated, though waste heat from the motor may in some cases be used to heat the airflow to a degree.
The third general type of hand dryer is the air-knife hand dryer, examples of which include the Dyson Airblade range of Hand Dryers manufactured by Dyson (UK) Limited and the Jet Towel range of hand dryers manufactured by Mitsubishi Electric Corporation
These hand dryers use an air-knife—effectively a sheet or curtain of moving air—to remove the water from the user's hands. The mode of operation is analogous to the established use of air knives in industry to remove debris or liquid from the surface of a product (see e.g. EP2394123A1, which describes removal of debris from a glass sheet using air knives): the air-knife moves across the surface of the hand and, as it does so, wipes or scrapes the water from the surface of the hand.
In both the Dyson Airblade and the Mitsubishi Jet Towel, two opposing, stationary air-knives are used, one for each side of the user's hand. The hands are inserted between the air-knives and then withdrawn slowly to effect the required relative movement between the hands and the air knives.
In the Dyson arrangement—shown in FIG. 1—the air knives are discharged through narrow, continuous slots (only the rear slot a is visible in FIG. 1), each less than 1 mm wide. In the Mitsubishi machine—shown in FIG. 2—the air-knife is instead discharged through opposing rows of individual discharge apertures (only the rear row b is visible in FIG. 2): here, the individual jets combine to produce the air knife downstream of the discharge apertures. In each case, the air knife is discharged at high speed (>80 m/s) to provide for an efficient wiping action across the surface of the hand.